Brothers volcano is a submarine dacitic caldera located on the southern Kermadec arc. It is host to the NW Caldera vent field (Site U1530 and Hole U1530A), which locally discharges more focused, metal-rich fluids, and the Upper Cone hydrothermal vent field (Site U1528 and Hole U1528D), which discharges predominantly diffuse, acidic fluids (pH 1.9). These two active vent sites were drilled in 2018 by International Ocean Discovery Program (IODP) Expedition 376. Fluid inclusions hosted in anhydrite, quartz, barite, and alunite recovered from drill core samples were studied by microthermometry, Raman spectroscopy, and laser ablation-inductively coupled plasma-mass spectrometery (LA-ICP-MS) to obtain detailed depth profiles of temperature, salinity, and composition of the hydrothermal fluids. These analyses allow for a better understanding of complex hydrothermal processes such as phase separation and an assessment of magmatic-hydrothermal contributions while making reference to the dynamics of the deep hydrothermal fluid that rises beneath the hydrothermal vents at Brothers.The fluid inclusions have homogenization temperatures (T_h) ranging from 149° to 358°C and salinities between 0.7 and 10.0 wt % NaCl equiv at the Upper Cone site and T_h of 254° to 394°C and salinities between 0.7 and 9.8 wt % NaCl equiv at the NW Caldera site. Microthermometry of fluid inclusions hosted in sulfate minerals from the NW Caldera site indicates subseafloor mixing between hydrothermal fluids and seawater. The enrichment of vapor-partitioning elements B and As in the fluid inclusions suggests phase separation subseafloor, which may be accompanied by halite dissolution and precipitation. Highly diverging Cl/Br values provide indirect evidence for halite dissolution that occurred via subseafloor convection of seawater. Petrographic observations made of the fluid inclusions, such as the recognition of combined liquid-rich and vapor-rich “boiling assemblages” and the occurrence of CO₂ in the inclusions, indicate phase separation of hydrothermal fluids.The CO₂ and the content of trace elements and metals in the fluid inclusions are significantly higher than that reported for Brothers vent fluid values, which reflects a magmatic-hydrothermal contribution. At the NW Caldera site, relatively high-temperature hydrothermal fluids with high Cu (max 560 ppm) and Zn (max 740 ppm) mixed with seawater before discharging at the sea floor. Depth profiles of the fluid inclusion data identify a few specific depths of channelized (focused) hydrothermal fluid flow. We suggest that the hydrothermal fluids are mainly focused along lithological contacts that act as permeable pathways, enhancing subseafloor hydrothermal fluid flow.

中文翻译：

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新西兰克马德克弧兄弟火山海底岩浆热液过程的流体包裹体证据

兄弟火山是位于克马德克弧南部的一个海底英安岩破火山口。它是西北火山口喷口场（U1530 点和 U1530A 孔）的所在地，局部排放更集中的富含金属的流体，以及上锥体热液喷口场（U1528 点和 U1528D 孔），主要排放扩散的酸性流体。 （pH 1.9）。国际海洋发现计划 (IODP) 第 376 号探险队于 2018 年钻探了这两个活跃的喷口点。通过显微测温、拉曼光谱和激光感应烧蚀研究了从钻芯样品中回收的硬石膏、石英、重晶石和明矾石中的流体包裹体。耦合等离子体质谱仪 (LA-ICP-MS) 以获得热液温度、盐度和成分的详细深度剖面。这些分析可以更好地理解复杂的热液过程，例如相分离，并评估岩浆热液的贡献，同时参考兄弟热液喷口下方上升的深层热液流体的动力学。流体包裹体具有均质化温度（上锥点的T _h ) 范围为 149° 至 358°C，盐度为 0.7 至 10.0 wt % NaCl 当量，而西北火山口的 T _h为 254° 至 394°C，盐度为 0.7 至 9.8 wt % NaCl 当量地点。对西北火山口遗址的硫酸盐矿物中的流体包裹体进行的显微测温表明，海底热液和海水之间存在混合。流体包裹体中蒸气分离元素 B 和 As 的富集表明海底存在相分离，这可能伴随着石盐溶解和沉淀。高度不同的 Cl/Br 值为通过海底海水对流发生的石盐溶解提供了间接证据。对流体包裹体进行岩相观察，例如识别出富含液体和富含蒸汽的“沸腾组合”以及包裹体中出现CO ₂，​​表明热液流体发生了相分离。CO ₂和微量元素的含量流体包裹体中的元素和金属明显高于兄弟喷口流体值报告的值，这反映了岩浆-热液的贡献。在西北火山口遗址，含有高铜（最高 560 ppm）和锌（最高 740 ppm）的相对高温热液与海水混合，然后排放到海底。流体包裹体数据的深度剖面识别了通道化（聚焦）热液流体流的一些特定深度。我们认为，热液流体主要集中在作为可渗透通道的岩性接触面，增强了海底热液流体的流动。